The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, the carbonaceous liquid product is called "shale oil".
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes the formation of a fragmented permeable mass of oil shale particles in a subterranean formation containing oil shale by undercutting a portion of the subterranean formation leaving unfragmented formation supported by a plurality of pillars. The pillars are removed, e.g., with explosive, and the unfragmented deposit is expanded to provide a permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying combustion zone feed into the retort on the trailing side of the combustion zone to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products and a residual carbonaceous material. The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniform fragmented permeable mass of formation particles having a reasonably uniformly distributed void fraction so gases can flow uniformly through the retort resulting in maximum conversion of kerogen to shale oil. A uniformly distributed void fraction in the direction perpendicular to the direction of advancement of the combustion zone is important to avoid channeling of gas flow in the retort. In preparation for the described retorting process, it is important that the formation be fragmented and displaced, rather than simply fractured, in order to create high permeability; otherwise, too much pressure differential is required to pass gas through the retort.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed, leaving the unfragmented portion supported by pillars. The pillars are then explosively expanded and after a time delay the unfragmented formation is expanded by detonating explosive placed in the pillars and in the unfragmented formation. This explosive expansion of pillars and unfragmented formation fills the void created by the undercut with a fragmented permeable mass of particles.
The general art of blasting rock formations is discussed in The Blasters' Handbook, 15th Edition, published by E. I. duPont de Nemours & Company, Wilmington, De.
U.S. Pat. application Ser. No. 833,240 filed Sept. 14, 1977 by Gordon B. French, titled EXPLOSIVE PLACEMENT FOR EXPLOSIVE EXPANSION TOWARD SPACED APART VOIDS, now U.S. Pat. No. 4,146,272, which is assigned to the assignee of the present application, describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids containing support pillars. The pillars are explosively expanded to spread the particles thereof uniformly across the void, and unfragmented formation adjacent the void is explosively expanded toward the void before overlying, unsupported formation can cave into the void. Said U.S. patent application Ser. No. 833,240 is incorporated herein by this reference.
Application Ser. No. 929,250 filed July 31, 1978, by Thomas E. Ricketts, entitled METHOD FOR EXPLOSIVE EXPANSION TOWARD HORIZONTAL FREE FACES FOR FORMING AN IN SITU OIL SHALE RETORT, now U.S. Pat. No. 4,192,554 describes the formation of a retort and recovery of liquid and gaseous products from the retort and is incorporated herein by reference.
It has been found that it is desirable to have an intact subterranean base of operation above a fragmented permeable mass of formation particles in an in situ oil shale retort. Such a base of operation facilitates ignition over the top portion of the fragmented mass, permits control of introduction of oxygen containing gas into the retort, provides a location for testing properties of the fragmented mass such as distribution of void fraction, and provides a location for evaluating performance of the retort during operation.
The base of operation is separated from the retort by a layer of unfragmented formation extending between the top boundary of the retort and the floor of the base of operation. This layer of unfragmented formation is termed a "sill pillar". It is important that the sill pillar which acts as a barrier between the in situ oil shale retort and the base of operation remain intact during retorting operations. During retorting operations the bottom of the sill pillar can be heated due to heat generated in the retorting operation, thereby causing sloughing of the bottom portion of the sill pillar into the retort. The sloughing of formation from the bottom of the sill pillar can weaken the sill pillar, thereby causing the sill pillar to lose structural integrity.
It can, therefore, be important to provide the sill pillar with support, thereby enhancing the probability that such a sill pillar will remain structurally sound during the retorting operation.